Cryogenic vacuum pumps (cryopumps) are widely used in high vacuum applications. Cryopumps are based on the principle of removing gases from a vacuum chamber by having them lose kinetic energy and then binding the gases on cold surfaces inside the pump. Cryocondensation, cryosorption and cryotrapping are the basic mechanisms that can be involved in the operation of a cryopump. In cryocondensation, gas molecules are condensed on previously condensed gas molecules. Thick layers of condensate can be formed, thereby pumping large quantities of gas.
Gases that are difficult to condense at the normal operating temperatures of the cryopump can be pumped at higher temperatures by cryosorption. In this case, a sorbent material such as activated charcoal is attached to the cold surface. The binding energy between gas particles and the adsorbing particle is greater than the binding energy between the gas particles themselves, thereby causing gas particles that cannot be condensed to adhere to the sorbent material and thus be removed from the vacuum system. When several monolayers of adsorbed gas have been built up, the effect of the adsorbing surface is lost and gas can no longer be pumped.
Cryotrapping can also be used to pump gases that are difficult to condense. In this case, the sorbent material is an easily condensible gas. The sorbent gas is admitted into the pump, forming a condensate on the cold surface. The difficult to condense gas is admitted at the same time and is adsorbed on the newly formed surface of easily condensible gas. A mixed condensate is thus formed.
Cryopumps are widely used for applications where contamination by nonprocess gases such as hydrocarbons must be avoided. Cryopumps typically use a closed loop helium refrigerator. Refrigeration is produced in a first stage operating at 50.degree. K. to 80.degree. K. and a second stage operating at 10.degree. K. to 20.degree. K. Conducting metal surfaces called cryoarrays are attached to the refrigerator stages and are cooled by them. Easily condensed gases, such as water vapor, argon, nitrogen and oxygen, are pumped by cryocondensation on the first and second stage cryoarrays. However, the lowest temperature achievable in a refrigerator cooled cryopump is so high (about 10.degree. K.) that not all gases normally present in a vacuum system can be pumped by cryocondensation. The gases which are difficult to condense, such as hydrogen, helium and neon, must be pumped by cryosorption. For this purpose, a sorbent material such as activated charcoal is attached to the second stage cryoarray. Further, only relatively low amounts of gas can be pumped by cryosorption, as only a thin layer (up to about 5 monolayers) can be formed on the surfaces. To pump large amounts of gas, a large amount of sorbent material must be used in the pump.
Small particles of the activated charcoal can break off the surface of the cryoarray, migrate through the cryopump to the vacuum chamber and onto the surfaces of the product being processed in the system, thereby contaminating the product. The contamination problem is particularly acute in connection with small, complex circuits being developed today, when semiconductor wafers are processed in the vacuum chamber. Particles of almost any size, including very small and fine size particles, are likely to produce defects in modern microminiature devices on semiconductor wafers.
The use of ion pumps in conjunction with cryopumps to enhance cryopump performance is disclosed by J. E. deRijke, "Performance of a Cryopump Ion Pump System", Journal of Vacuum Science and Technology, Vol. 15, No. 2, March/April 1978, pages 765-767. A standard cryopump with sorbent charcoal on the second stage and a standard noble gas ion pump were used to increase total pumping speed and total capacity of gas that can be pumped before regeneration was needed. The disclosed configuration did not address the problem of vacuum system contamination by charcoal particles.
A turbomolecular vacuum pump having a heat exchanger located in its suction port is disclosed in U.S. Pat. No. 4,926,648 issued May 22, 1990 to Okumura et al. The heat exchanger is connected to a refrigerator through a refrigerant pipe. The refrigerant is cooled from about -100.degree. C. to about -190.degree. C. and is used to condense water vapor.
Tests to measure the effect of cryotrapping of hydrogen by argon in a cryopump without the use of charcoal are described by R. C. Longsworth et al, "Cryopump Vacuum Recovery After Pumping Ar and H.sub.2 ", J. Vac. Sci. Technol. A, Vol. 9, No. 5, Sept./Oct. 1991, pp. 2768-2770.
A cryopump having sorption surfaces of reticulated vitreous carbon attached to the second pumping stage is disclosed in U.S. Pat. No. 4,791,791 issued Dec. 20, 1988 to Flegal et al.
It is a general object of the present invention to provide improved methods and apparatus for vacuum pumping an enclosed chamber.
It is another object of the present invention to provide methods and apparatus for vacuum pumping with a cryogenic vacuum pump wherein the potential for contamination of the vacuum chamber by a sorbent material is eliminated.
It is a further object of the present invention to provide methods and apparatus for vacuum pumping an enclosed chamber wherein a cryogenic vacuum pump is used with an auxiliary pumping device that removes gases which are difficult to remove by cryocondensation or cryotrapping.
It is a further object of the present invention to provide improved methods and apparatus for vacuum pumping a plasma vapor deposition chamber.